Wireless communication systems are widely used to provide various types of communications. For example, voice and/or data are provided by the wireless communication systems. A conventional wireless communication system provides multiple users with one or more shared resources. For example, the wireless communication system can use various multiple access schemes such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA).
An orthogonal frequency division multiplexing (OFDM) scheme uses a plurality of orthogonal subcarriers. Further, the OFDM scheme uses an orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter transmits data by performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of subcarriers, and the receiver uses FFT to split the plurality of subcarriers. According to the OFDM scheme, complexity of the receiver can be reduced in a frequency selective fading environment of a broadband channel, and spectral efficiency can be improved through selective scheduling in a frequency domain by utilizing channel characteristics which are different from one subcarrier to another. An orthogonal frequency division multiple access (OFDMA) scheme is an OFDM-based multiple access scheme. According to the OFDMA scheme, a radio resource can be more efficiently used by assigning different subcarriers to multiple users.
Recently, to maximize performance and communication capability of the wireless communication system, attention is paid to a multiple input multiple output (MIMO) system. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas in order to improve efficiency of data transmission and reception. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then assembled into one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can increase with respect to a specific data transfer rate.
Hereinafter, downlink is defined as a communication link from a base station (BS) to a user equipment (UE), and uplink is defined as a communication link from the UE to the BS.
In general, the BS schedules uplink and downlink radio resources in the wireless communication system. User data or control signals are carried using the uplink and downlink radio resources. A channel for carrying user data is referred to as a data channel. A channel for carrying control information is referred to as a control channel. A physical layer channel for transmitting the control information from the BS to the UE is referred to as a physical downlink control channel (PDCCH) or downlink layer 1/layer 2 (DL L1/L2) control channel. A physical layer channel for transmitting the control information from the UE to the BS is referred to as a physical uplink control channel (PUCCH) or an uplink layer 1/layer 2 (UL L1/L2) control channel. The control information transmitted through the PUCCH may be various, e.g., a scheduling request signal for requesting radio resource allocation, an acknowledgement (ACK)/negative-acknowledgement (NACK) signal that is a response for transmission of downlink data, a channel quality indicator (CQI) indicating downlink channel quality, a rank indicator (RI) that is MIMO-related control information, a precoding matrix indicator (PMI), etc.
The BS schedules the uplink and downlink radio resources according to the control information received from the UE, and reports scheduling information to the UE through the PDCCH. The BS may use the RI and the PMI transmitted by the UE without alteration. The RI and the PMI may be determined and used by the BS. The BS has to report the RI and the PMI used in radio resource scheduling to the UE through the PDCCH. However, the PDCCH is limited in size, and there is also a limit when a system using 4 Tx antennas transmits the RI and PMI with a size of 6 bits through the limited PDCCH. That is, since the RI and PMI, which can be transmitted for multiples users through the PDCCH during one transmission time interval (TTI), are limited, a multi-user scheduling gain cannot be obtained.
Accordingly, there is a need for a method for effectively transmitting MIMO-related information to a UE in a multiple antenna system.